Cis- and trans-5-methyl-2-cyclohexenyl chloride (1a and 1b) both react in acetic
ID: 548045 • Letter: C
Question
Cis- and trans-5-methyl-2-cyclohexenyl chloride (1a and 1b) both react in acetic acid (HOAc) containing sodium acetate (NaOAc) to give the corresponding 5-methyl-2-cyclohexenyl acetates, 2a and 2b 3. CH CO H CH,CO,Na HjC OAc la: cis lb: trans 2a: cis 2b: trans The following facts about the reaction are known: (1) The rate of product formation was found to be first order in [1a] or [1b] and zero-order in [NaOAc]. (ii) The same 2a:2b product ratio (5:95) was obtained whether 1a or 1b was the reactant. (ili) Starting with optically active 1a or 1b, the acetate products 2a and 2b were both ound to be racemic. (iv) Acetates 2a and 2b were both found to be stable under the reaction conditions (i.e. they do not interconvert) (a) Propose a reaction mechanism consistent with the above observations. Clearly explain how your hypothesis is consistent with each observation. (b) A kinetic study determined the rate of product formation and the rate of loss of optical activity during the reaction with optically active 1a as reactant. Interestingly, it was found that the latter rate was ca. 3 times greater than the former. To investigate the origin of this result, the reaction was stopped at partial conversion and the chloride was isolated from the reaction mixture. The recovered chloride consisted of only 1a, however, its optical rotation was found to be only ca. 50% of its original value. Modify your mechanistic proposal from part (a) to account for these observations. Clearly explain how your hypothesis is consistent with each observation.Explanation / Answer
a).The reaction rate is consistent with the Ist order kinetics. Thus the rate only depends on the substrate concentration and the Zero order with respect to the acetate concentration. Hence it can be confirmed that the first reaction is following the first order kinetics from the kinetics data.
The Trans product is formed predominantly dure to the higher stability with respect to the cis product. The trans is more stable as it has one substitutent in the equatorial positiona and the other substituent in the axial position. Thus negating the 1,3 diaxial strain. The mechanism is the SN1 reaction pathway.
b). The cis product is prediminant in the reaction if its stopped midway. The reaction has the intermediate of the carbocation and then the product is product. THe cis product is found predominantly in the isolated product due to the